1. Field of the Invention
The present invention concerns electronic picture tubes and, more precisely, electron beam deflectors associated with these tubes.
Electron beam tubes concerned by this invention are electron beam tubes in which the deflection of the beam is used either to depict the picture on a display screen, in the case of cathode-ray tubes (CRTs), or to pick up the picture in the case of camera tubes.
2. Description of the Prior Art
It will be recalled that a CRT is formed by a vacuum chamber in which there is an electron gun to form an electron beam, and a display screen that emits light when it is struck by the beam. Means for modulating and deflecting said beam enable the picture to be depicted. These means may be located inside or outside the vacuum chamber.
The electron beam may be modulated or deflected by electromagnetic fields. Said means of deflection and modulation are therefore, generally, devices to create appropriate electromagnetic fields to obtain the desired picture
While the modulation and electrostatic deflection are often obtained by means of electrodes taken to high voltages and placed within the tube, the electromagnetic deflection is generally obtained by means of magnets, electromagnets or coils mounted outside the tube.
The geometrical precision of the image depends on the precision of modulation or deflection obtained by the means used, and may be spoiled by stray electromagnetic fields coming from the environment of the tube when it is set up in a piece of equipment. For this reason, CRTs designed to work in complicated environments are generally surrounded by an electromagnetic shielding device designed to prevent the surrounding electromagnetic fields from penetrating the tube and thus avoid disturbances in the picture.
This electromagnetic shielding device may be a casing made of a material that is a good conductor of electricity and has high magnetic permeability. Electromagnetic shielding means such as this are commonly used for CRTs placed on board aircraft, tanks, ships or helicopters, for example.
In certain improved CRTs of the prior art, corrections are made to the geometry of the picture by means of electromagnetic fields given by permanent magnets. The tunings of these correction fields are done in the factory. These tuning operations consist in positioning the permanent magnets so as to obtain the desired picture geometry. To be effective under the operating conditions of the CRT, these tunings must be made under similar electromagnetic environmental conditions, namely the CRT should be surrounded by its electromagnetic shielding during these correction field tuning operations. This is because, as we know, the magnetic fields are disturbed by the metallic masses that surround them. Hence, the picture geometry correction obtained depends on the relative position of the magnets and of the shielding: this positioning becomes critical for the quality of the picture.
This point entails several drawbacks for the industrial scale manufacturing of the assemblies of shielded CRTs, among them the drawbacks indicated here below. Since the manufacture of CRTs comes rather within the field of glass technology while that of the shielding comes rather within the field of plate work, the tube manufacturer often entrusts the making of the latter to a sub-contractor. The CRT manufacturer must have the shieldings available in order to make the correction tunings, and must therefore keep them in store. If a shielding of a different geometry is used, the correction field of the magnets has to be modified. A deflector designed for one shielding cannot be used for another shielding of a different shape. This means that the manufacturer has to manufacture different shieldings for each type of CRT, hence often for a large number of different CRTs. Also, there will be as many different deflectors as there are different CRTs and different shieldings.
If the deflector and the shielding are not perfectly aligned, the "shunt" effect of the electromagnetic field of the magnets due to the metal of the shielding is dissymmetrical, and this introduces a distortion into the picture.
The aim of the present invention is to overcome these drawbacks by proposing a deflector, with field correction by permanent magnets, that is sensitive neither to the shape nor to the relative geometry of assembly of the electromagnetic shielding.
The device according to the invention makes it possible to avoid the need for lengthy tuning operations in which additional magnets are added on as well as for tuning operations that are individually adapted to the shieldings. The device according to the invention enables the use of practically any geometry and shape of shielding and therefore enables a considerable reduction in the number of different deflectors used by a CRT manufacturer.